Basic Theory of Turbo Charging
An engine fundamentally creates power by mixing fuel and air together and then igniting it. The bang resulting from this is used to rotate the piston. The problem with this is that the engine can only ingest a set amount of air per piston stroke. This limits the amount of fuel that can be burned and ultimately limits the power that can be produced.

There are two ways of overcoming this:-

Make the engine move more air. This can only be done by increasing the rpm of the engine. As the pistons move up and down faster the engine uses more air, more fuel can be mixed in and more bangs
per second can happen. This increases the amount of horsepower the engine produces (horsepower is a measure of the amount of work done in a given time)

Physically force more air into the engine. Without increasing the revs, air can be forced into the engine under pressure. This allows more fuel to be added and creates a bigger bang. This method is known as forced induction and turbo charging is one way to do this

ST205 turbo system

Theory of Turbo OperationTurbines and Compressors
Exhaust gas exits the engine and feeds via the exhaust manifold into the turbine side of the turbo. Here it acts on the Turbine Wheel causing this to rotate (at speeds in excess of 100,000RPM).

The turbine wheel is directly connected to the compressor wheel which is situated in the engine air intake system. When the turbine wheel spins this also spins the compressor wheel and this in turn compresses the air in the inlet system creating boost. As engine rpm rises more exhaust gas will be produced. This will spin the turbine and compressor wheels faster and faster and produce more and more boost. It is also worth noting that as boost levels rise more exhaust gas will be produced too. So boost will rise faster than engine rpm.

Compressor Wheel

Left to it's own devices the turbo will produce far more boost than the engine can cope with, so
something has to be done to limit this boost before the engine expires:

Waste Gate Valves
The way to limit boost in a turbo system is to prevent all of the engine exhaust gasses passing through the turbine side. This will limit the turbine and compressor wheel speed and ultimately the boost pressure in the intake. This is done with the waste gate valve.

When the valve is closed - all of the exhaust gas flows through the turbine spinning the wheel as fast as possible.
When the valve is open - some of the exhaust gas bypasses the turbine reducing the speed of the turbine wheel and directly reducing boost pressure

Turbine and Wastegate

Actuators
So now there is a way of producing boost and limiting it. But you need a control mechanism to make the boost producer (turbocharger) and boost regulator (wastegate valve) work together to make a constant boost pressure - this is the job of the actuator.

The actuator is essentially a boost controlled switch. It is basically a diaphragm attached to a control arm which is directly connected to the wastegate. One side of the diaphragm is exposed to the boost pressure in the inlet manifold. Once boost pressure exceeds a certain base level (known as actuator pressure) the diaphragm will start to deflect. This will cause the control rod to start opening the wastegate, lowering boost pressure. It is a self regulating system. The more boost the turbo created the more the actuator will open the wastegate reducing boost

This state is actuator controlled boost and without changing the actuator the amount of boost pressure cannot be changed. Or can it?

Increasing Boost Pressure
In an actuator controlled system it is possible to raise boost pressure. The key is to reduce the amount of boost pressure the actuator is exposed to. This can be done by allowing air to escape from the actuator in a controlled fashion. The escaping air will reduce the apparent pressure in the wastegate meaning that the intake system must be pressurised more to achieve base opening pressure in the actuator. Hey presto, you have raised the intake pressure without increasing the base actuator pressure.

In the GT4 system this is done with the VSV (Vacuum Solenoid Valve) system and it is done in different ways on different models...

In the ST165 and ST185 models the VSV is a simple system. It consists of (in simple terms) a fixed size nipple which will bleed off a set amount of pressure. This bleeding is controlled by a signal from
the ECU. When the VSV is switched off the actuator is exposed to full manifold pressure. When the VSV is switched on a fixed amount of pressure is bled off (0.2 bar) which raises the intake pressure by 0.2 bar.

This is a crude but effective system. Variations in the actuator base pressure will result in identical variations in the inlet manifold pressure. The ECU cannot control this since it can only bleed off a fixed amount of pressure. So as the cars get older and the wastegates begin to loose stiffness the boost start to drop.

The ST205 model uses a different system. It has a VSV which is capable of bleeding off much more pressure (tests seem to show that it can bleed 0.5+ bar). If the VSV is permanently enabled then most ST205s will exceed 1.3 bar of boost. However, the ECU does not just switch the VSV on or off like the earlier models. Instead it uses a closed loop system to switch the VSV on or off based on boost pressure. The VSV is
basically driven by a pulsed signal so that it is on some of the time and off some of the time. On average this switching will lower the amount of boost pressure which is bled from the actuator. So the ECU
monitors the inlet manifold pressure and varies the on/off switching time of the VSV to ensure that the correct manifold pressure is maintained. This system is also (within reason) un-effected by degradation
of actuator base pressure

Aftermarket Electronic Boost control
This is generally done with a solenoid as on the ST205 model. However the arrangement is done slightly differently. Rather than bleeding pressure away from the wastegate the solenoid is usually fitted between the turbo and the wastegate actuator. When the solenoid is closed the wastegate is not exposed to boost pressure and boost is unlimited. When the solenoid is open the wastegate actuator is exposed to full boost pressure and boost is limited to actuator pressure only.

The boost controller rapidly switches the solenoid on and off (open and closed) exposing the wastegate to some of the boost pressure but not all of it all the time. By altering the ratio of solenoid open time to solenoid closed time (the duty cycle) the average pressure that the wastegate can be controlled.

In most aftermarket boost controllers the operator can either manually set a duty cycle or set a target boost and let the EBC control the duty cycle.

Apexi AVC-R Boost Controller

Dump Valves (Blow Off Valves)
So now we have a turbo that produces lots of controlled boost there is one last problem to overcome. What happens when the engine does not want boost any more? This happens all the time in the real world. Every time you change gear the butterfly in the throttle body closes completely leaving the turbo spinning at 100,000 rpm generating boost with nowhere for it to go. The result is that the compressor stalls quite violently as the air it's compressing suddenly stops flowing. This puts a significant shock loading on the shaft connecting the turbine and compressor.

One way of overcoming this is a dump valve. It connects to the inlet system after the turbo and allows the turbo to vent excess pressure

It works like this :-

The Dump Valve has a diaphragm (or a piston in an aftermarket setup) with a chamber either side. The "top" chamber is attached to the inlet manifold and the "bottom: chamber is attached to the inlet system before the throttle body. When the engine is turned off both sides of the diaphragm are at the same pressure and the elasticity of the diaphragm holds a piston arrangement shut preventing
airflow through the mechanism. This is also true when the engine is on boost. With the throttle body butterfly open the exact same pressure is applied to the diaphragm. However, if you suddenly close the throttle butterfly (as during a gear change) the turbo is producing boost but the inlet side is in vacuum as air cannot pass the butterfly. This creates a pressure imbalance in the dump valve. The "top" chamber is in vacuum while the bottom is under pressure. This lifts the plunger mechanism off of its seat and allows air to flow through the valve back into the air box.

Bailey Dump valve

HKS Super Sequential Dump Valve

The ST205 is the only model that has a dump valve as standard and fitting one to other models can cause running problems unless it is done correctly. On these models the dumped air needs to be re-circulated back into the inlet system after the airflow meter but before the turbo.

Intercoolers
The process of compressing air in the inlet manifold with a turbocharger will also add a great deal of heat into the air. This is far from desirable. As the air gets hotter it becomes less dense and combusts less and less well.

In order to combat this the output from the turbo is fed through an intercooler. This basically passes the hot air through a radiator where some of the excess heat is removed from it, cooling the inlet charge fed
to the engine and allowing for maximum power.

Intercoolers come in two basic varieties and a whole host of different variations on these themes

Air To Water Intercoolers
Air to water intercoolers are also referred to as chargecoolers. This system is used on the ST165, ST185CS/RC and the ST205. It is quite a simple system but it does have some problems

ST165

ST205

Chargecoolers use a water circuit to cool the air which comes out of the turbo. Essentially this air passes through a large sealed box (which can clearly be seen on top of the engines). Inside this box there is a large finned radiator which the hot air passes across. Running through this radiator is fluid which is heated as the hot air passes over it cooling down the air (or charge as it's known). To combat the heating of the fluid another radiator is fitted to the front of the car. Fluid from the chargecooler is circulated through this additional radiator to keep it cool. Overall the cool water removes heat from the compressed air and then the second radiator removes heat from this water

The major advantage of this system is that it keeps the path from the turbo outlet to the inlet manifold very short. This means that the dreaded turbo lag is kept to a minimum.

This system is fine but it does have limits. First of all there are two in-efficient exchanges of heat. One from the hot compressed air to the water cooling system and a second from the heated cooling water to the air outside the car. Secondly, sitting on top of the engine the cooler is also prone to heat soak when stationary for long periods. This is somewhat detrimental to overall performance

It is also worth noting that all of the stock systems are limited in their overall flow capacity. They were designed to work with a standard engine and while they will cope with increased boost pressure and flow there is a limit.
Rumour has it the ST205 unit ceases to be effective somewhere close to 400HP so those aiming for figures above this should consider an upgrade.

Track testing has also shown that charge temperatures begin to climb alarmingly after 10-15 minutes of hard driving so those on a track biased mission should also seriously consider upgrading even with a standard engine

Air To Air Intercoolers
These are probably the more common intercooler system. The hot post turbo compressed air is fed into a large finned radiator which is directly cooled by airflow from outside the car.

ST185

This system is used on the ST185 as standard but in a flawed application. An air to air intercooler must have cold air flowing through it for it to be effective. Without airflow the hot turbo air will soon make it very hot and ineffective. In the ST185 setup the air to air cooler has two problems

First of all it uses a quite small bonnet vent to duct air over the IC core. This means that the flow of cold air is sub optimal and the core heats up more than it should do. Secondly the IC core is located on top of a very hot engine. This means that when airflow is slow (i.e. car not moving very fast or even worse stationary) the IC core rapidly reaches engine temps offering little to no cooling of the inlet charge.

The more optimal air to air system is the front mount setup commonly known as a FMIC. In this system the IC core sits right behind the front bumper. It is also generally considerably larger than the stock ST185 ATA core. This means that it has a high flow capacity and a much higher cooling capacity than any of the stock systems, be it air to air or water to air. Also, since the IC core is well away from the engine heat it does not suffer the heat soak problems which afflict all of the stock GT-Four setups. There are only two real disadvantages. Firstly the pipe runs are longer which can cause some additional turbo lag. Secondly the actual routing of the front mount pipe work is awkward since the stock car is not set up for a front mount system